(1) Field of the Invention
This invention relates to the deposition of hard chrome coatings from plating solutions. More particularly, this invention relates to the deposition of hard chrome coatings by means of brush plating.
(2) Prior Art
A number of coatings are deposited from so-called plating baths in which a coating solution is subjected to an imposed electrical potential. Such imposed electrical potential basically enhances an already naturally occurring tendency for any metal ions in a solution to be deposited, or plated out of solution, upon any metal object or surface immersed within or partially within the solution. Such metal surfaces are, under favorable conditions, able to supply electrons to metallic ions dissolved in the solution converting such ions to less soluble metallic atoms which are deposited upon the electron donor material. This natural deposition, or plating out, of the coating material from a natural solution may be rather slow or in many cases even more than counterbalanced by simultaneously proceeding resolution processes. However, the natural deposition or plating rate can be improved dramatically by application of an external electrical potential to a plating bath, in effect causing a current to flow through the bath, such current serving to rapidly convert dissolved metal ions to metal atoms which deposit or plate out as a coating on the cathode from which electrons are derived. Such externally applied current also more quickly forms metallic ions at the anode when appropriate which ions dissolve within the solution of the coating bath to take the place of those deposited or plated out upon the cathode or other adjacent materials. So-called "electrolytic coating" using electrolytic coating baths is very widely used, both on a small scale and very large scale for production-type coating facilities.
While conventional coating baths are effective and efficient means for the coating of metal bases such as iron and steel and the like, the large tanks of solution necessary to effect a normal coating from an electrolytic coating bath make the process practical only for fairly large permanent installations. There is frequently a necessity, however, to conduct plating operations in emergency or job shop-type situations, on relatively small pieces or sections or single items or objects of metal, or upon items in the form frequently of single broken or worn metal apparatus which needs to be refurbished by replating or the like. Emergency repairs, for example, may be conducted on shipboard or other places where the provision of a full-scale or even a relatively small scale plating bath is either impossible or at best impractical.
So-called brush plating is an alternative to tank plating, and in some cases, a preferred method of plating. This process, which is generally known as brush plating in the trade at large, is also known by a number of other names, and particularly by plating experts as "selective plating", not to be confused with "selective plating" accomplished in a tank, frequently on copper-based alloy contacts and usually also with gold plating. Selective plating in a plating tank environment is usually accomplished by thoroughly masking all parts not to be plated. Selective or brush plating is generally a more convenient, although to some extent more difficult, process to effectively use than tank plating. There have been a number of recent additional names suggested for brush plating. Among such names are "stylus plating", "contact plating", electrochemical metalizing", as well as "selective electrochemical metal deposition", sometimes known by the acronym "SEMD". The common name among plating experts, however, as pointed out above, is "selective plating". The term brush plating, used commonly in plating shops, is descriptive, since the basic principal of the technique is to continuously agitate or abrade the surface being plated to remove bubbles of hydrogen which bubbles may otherwise collect upon the surface and interfere with the efficiency of the plating. The term brush plating will consequently be used in this description.
Brush plating has a number of advantages over tank plating, of which the following may be particularly mentioned: (a) first and most important, is the ability of brush plating to deposit an electroplated metal precisely onto the portion of the surface of a base metal where it is desired in almost any thickness which is desired. In fact, this is the origination of the name "selective plating"; (b) secondly, in some cases brush plating or selective plating can provide superior coating at a cheaper price;. (c) thirdly, brush plating or selective plating can be used in environments where normal vat or tank plating will not be available, for example, at the location where a repair must actually be made, for example, on shipboard and in other locations where a substantial vat of coating solution would not be available.
Brush plating, or selective plating, has become particularly popular for repairing previously coated surfaces where only a portion of such surface has been seriously worn or is otherwise damaged such as, for example, on rotatable shafts and the like where continuous movement of the shafts may have worn through a previous coating in a particular portion or otherwise seriously eroded the surface. For the same reason, brush plating, or selective plating, can often be used to fill in a discontinuity which has developed in the surface of another metal piece even where such original piece was not coated. Again, therefore, brush plating or selective plating is particularly valuable in repairing or refurbishing worn materials such as shafts and the like which are subject to severe localized wear due often to breakdown in their normal lubrication or to an unequal or unbalanced operation or the like.
Brush plating, or selective plating, can be accomplished either by sophisticated apparatus made especially for such plating or can be a hand operation using only very basic apparatus, the movement of the "brush" portion of which is accomplished manually. Basically, in the usual process, a graphite or sometimes platinum anode, which may be either mechanically supported and actuated or hand held and which is designed to conform to the shape of the work piece which is to be repaired or coated, is held or supported close to the surface of such work piece while a current is passed through a plating solution continuously between the anode and the surface to be repaired or otherwise treated. The anode is maintained positive and the work piece is given a negative charge via a suitable negative contact which converts the work piece into the cathode. When the anode and work piece are brought close together with appropriate plating solution between the two and with current passing from the cathode to the anode via the metallic ions of the bath, metal ions from the plating solution are deposited upon the work piece opposite the anode, or those portions of the anode which most closely approach the surface of the work piece. This enables the size and shape of the anode to determine the size and shape of the area to be coated. The anode must never touch the surface of the work piece else all the electric charge would arc between the anode and cathodic work piece melting any coating already plated out and usually damaging not only the work piece itself, but also quite likely the anode as well.
Since it is difficult to merely pass or flow plating solution between an anode and a work piece without the area in question being surrounded by a container of some sort, the anode is usually provided with an absorbent material upon its surface which will temporarily retain the plating solution. The absorbent material is held close to the surface of the work piece to maintain the work piece continuously subject to or bathed within the plating solution absorbed in or saturating such absorbent material.
The absorbent material should be formed from a dielectric material to prevent arcing between the anode and cathode. Some form of special abrasive or rubbing material is also frequently affixed to the face of the anode or to the absorbent material to rub on the surface of the work piece in the area to be plated as the anode passes over it. Such abrading or rubbing serves to displace the bubbles that normally form on the surface of the work piece in any plating operation, which bubbles may partially shield the work piece surface from the plating solution, thus interfering with the plating operation. Rapid removal of such bubbles, usually comprising hydrogen, enables a more rapid, uniform and effective plating to be accomplished. If a special rubbing or brushing material is not used, the surface to be plated is rubbed with the surface of the absorbent material which serves to remove the hydrogen bubbles formed upon the surface as plating proceeds.
In order to continuously abrade or brush the surface of the work piece during the coating operation, either the graphite anode or the cathodic work piece is maintained in substantially constant motion. Where a round section such as a shaft is being coated, it is usually most convenient to rotate such shaft with respect to the anode, while for other shaped pieces and particularly the usual flat work surface, it is usually found more convenient to move the anode continuously during the brush plating process. The brush-plating process is frequently used to restore both the outside diameter and the inside diameter of cylindrical objects as well as the configuration of plane surfaces of many parts such as, for example, shafts, bearings, hollow members, journals and other work pieces, to an original dimension or to provide specific surface conditions, usually wear-resistant surfaces. The brush coating process is also often used for the filling of corrosion pits and the like in metal surfaces and in providing hard facing and the like upon metal surfaces.
In general, brush plating, or selective plating, coatings are usually more dense and fine grained as well as less porous than similar coatings applied by other types of electroplating processes. It has been said that brush plated coatings are, in general, seventy-five percent less porous than deposits formed by tank plating and ninety-five percent less porous than deposits applied by metal powder or wire spray-type coating processes. Because of such additional denseness, the deposits frequently offer greater corrosion resistance as well as hardness. The final results, however, depend largely upon the metals used as coating materials and the coating process.
Brush or selective plating usually provides, as indicated briefly above, much harder as well as denser deposits than those obtained by other types of electroplating so that brush-plated work pieces are usually more abrasion resistant and less susceptible to fatigue loss during use.
Superior adhesion of the coating material is often also attained by a properly operated brush or selective plating process. This is believed to result, however, more from the fact that an organic plating solution is usually used in a brush-plating operation, whereas an inorganic solution is frequently used in tank plating and other general electroplating operations. Organic plating solutions generally have a higher conductivity than inorganic solutions and therefore the work piece is customarily subjected to a far greater current density, often in the range of 1000 to 3000 amps per square foot rather than the 100 to 500 amps per square foot which is more customary in tank plating arrangements. Such high current density is almost equivalent to an arc welding process and the metal ions therefore seem to be driven more forcefully into microscopic valleys and cracks upon the surface structure of the base metal, locking them more effectively into place. In conventional tank plating, on the other hand, the plated coating often seems to merely plate over valleys, cracks and other inequities in the surface of the work piece. The close spacing between the anode and the cathodic work piece and the fact that the charge on the anode is not dissipated by dissolution or dissolving of anode material into the plating bath to replenish the metal ion content of the bath also probably has considerable to do with the more intimate coating produced.
Many metals can be successfully brush plated, including cadmium, cobalt, copper (both from acidic and alkaline solutions), gold, nickel (both from acidic and neutral plating solutions), rhodium, silver and tin. One notable and well-known failure of brush plating, however, has been the inability to provide a hard chrome deposit by brush plating even though brush plated deposits are usually more dense than equivalent tank plated coatings. While extremely thin hard chrome coatings have been sometimes attainable and generally thin, relatively soft deposits of chrome could be attained heretofore using brush plating techniques, thicker hard chrome deposits were completely unattainable. As may be imagined, this lack of ability to form hard chrome coatings has been a serious drawback since hard chrome deposits are, in general, superior to any other electroplated surface for wear resistance, low coefficient of friction, hardness, heat resistance and non-galling characteristics. In view of this, sometimes nickel has been plated in place of chrome and a nickel tungsten or nickel cobalt alloy has also sometimes been used in place of a hard chrome plating to take advantage of the preciseness, affordability and other conveniences of the brush-plating process.
A number of efforts have been made to successfully deposit hard chrome coatings or thicker hard chrome coating using the brush-plating process. However, to date, no successful process or apparatus for plating with hard chrome has, so far as the present inventor is aware, been developed. Furthermore, no adequate theory to explain the inability to provide hard chrome coatings by brush plating has been advanced. While very thin hard chrome coatings have been made, it has been impossible to provide useful thicker hard coatings. There has been a need, therefore, for a brush-plating process which can successfully provide a hard chrome surface coating of reasonable thickness. Some of the more pertinent prior art patents related to the problem of brush plating of hard chrome coated surfaces or having disclosures showing the state of the art or otherwise of interest in this regard are as follows.
U.S. Pat. No. 3,751,343 issued Aug. 7, 1973 to A. J. Macula et al. discloses a hand tool for brush coating metal surfaces with an increased rate of deposition. Macula et al. discloses that brush coating apparatus at the time of the filing of his application was usually in the form of a hand tool which was rubbed or brushed against the surface to be plated as electrolytic action took place. The hand tool which served as the anode of the coating operation was wrapped in a porous or dielectric fabric sock saturated with the coating solution or electrolyte and rubbed over the surface during the process of coating. Macula felt that the inability of the brush-coating process known in his time to operate at high current densities limited the rates of metal deposition and his solution was to initiate movement of the porous sock with respect to both the cathodic work piece as previously practiced, but also with respect to the anode at the same time. Macula's improvement, therefore, was to provide a combined rubbing action on both the anode and the cathodic work piece at the same time. Such rubbing, he believed, removed unwanted products of electrolysis as well as avoided passivation and polarization of the anode and cathode as well as the usual physical removing of gases and unwanted precipitates from the surface to be plated. The porous electrolyte-saturated sock of Macula could be made of various fabrics, for example, cotton, flannel, felt, canvas, but was preferably made of resinous materials such as dacron-polyester fibers, which he used especially for chromium plating, since such materials, according to Macula, had good resistance to attack by chromic acid. So far as is known, the Macula process was not effective for forming hard chrome coatings.
U.S. Pat. No. 3,001,925 issued Sep. 26, 1961 to E. V. Berry discloses an anode structure for an electrolytic coating bath for coating sections of a crank shaft which may be rotated within the coating bath. The arrangement, which is a more or less conventional coating bath and not a brush plating arrangement, is said to provide rapid deposition of hard chromium coatings. Berry makes use of a lead anode which at least in part closely surrounds the portions of the work piece to be chromium coated. Such anodes are made either of a lead-antimony or an alloy of lead and tin and are curved so they pass partially about the surface of the portion of the crank shaft to be coated, but preferably not completely about it, leaving open either a lower or top portion. The lead anode itself is contained within a holder which shields it from contaminants in the bath. The surface of the anode is preferably grooved or ridged in order to provide an increased ratio of anode-to-cathode surface which Berry states has been found to decrease the formation of trivalent chromium in the chromic-acid bath. Berry goes on to indicate that an increase in trivalent chromium in a chromic-acid plating bath is highly undesirable because it increases the electrical resistance of the bath which is also increased with an increase in temperature of the bath. Berry also discloses that after prolonged use, the surfaces of the lead anodes become covered with oxide and chromate coatings which are relatively poor conductors of electrical energy and therefore form insulators over the surface causing an undesirable rise in the temperature of the bath. Berry tries to arrange for an evolution of oxygen bubbles for continuous removal of hydrogen gas from the various surfaces, but apparently was not particularly successful in this endeavor. It should be emphasized that the Berry patent is directed specifically to tank plating and not to brush plating and generally illustrates, as an example, the only viable practical type of arrangement for plating hard chrome coatings upon work pieces available in the past.
U.S. Pat. No. 4,269,686 issued May 26, 1981 to A. W. Newman et al. also discloses an apparatus for electroplating the bearing surfaces of a crank shaft within a plating bath as distinguished from brush coating. The plating bath of Newman is a chromic-acid bath for plating chromium on the bearing surfaces. Newman discloses an arrangement for his anodes to closely encompass the surfaces of the crank shaft to be plated without touching such surfaces and discloses that such anodes should be formed from a lead composition.
As indicated, the Newman patent is directed to tank coating or plating and does not provide any way to brush plate chromium. Newman, consequently, has the disadvantages of tank plating, particularly its lack of ready portability and inconvenience in the repair of limited portions of defective work pieces.
U.S. Pat. No. 4,359,366 issued Nov. 16, 1982 to C. D. Eidschun discloses an arrangement for brush plating printed circuit boards in an electrolytic plating bath. The use of natural polypropylene is disclosed within the anode chamber. While the Eidshun patent is essentially a brush-plating procedure, the brush used is a stainless steel conductive brush. The patent is limited basically to salvaging misplated circuit boards and would appear to have little other application.
U.S. Pat. No. 4,452,684 issued Jun. 5, 1984 to K. Palnik discloses a brush or selective plating apparatus said to accomplish high speed selective plating by the brush method using a brush comprised of a molded body formed with a porous, hydrophobic material covered by a felt-like material. A porous platinum sheet or screen is positioned between the two to serve as the anode. The electrolytic solution is distributed through a conduit located interiorly of the brush and passes outwardly through small pores in the hydrophobic material till it covers the felt-like material. A suitable porous hydrophobic material is disclosed by Palnik to be preferably a molded polypropylene having pores uniformly dispersed throughout so that it is pervious to liquids. Palnik states as a generalization that larger pores and greater pore density will permit faster plating rates but may result in more plating solution being deposited on the surface of the material to be plated than necessary, making selective plating more difficult to control. In the arrangement of Palnik, the parts to be coated are passed by the stationary brush material in contact therewith only once rather than being subjected to multiple passes or a back-and-forth rubbing or abrasion. Palnik's arrangement is designed to be used only for the plating of gold and other precious metals on electrical contact apparatus and the like. Recirculation of plating solution and replenishment of spent solution prior to or during recirculation is broadly disclosed, but not detailed.
U.S. Pat. No. 4,610,772 issued Sep. 9, 1986 to K. Palnik also discloses the use of a porous hydrophobic material having interconnected pores made from molded porous polypropylene as disclosed in Palnik's earlier patent. In the '772 patent, however, Palnik provides for the use of a rotating rather than a stationary brush. Palnik prefers in both cases to use a porous polypropylene brush material having pore sizes in the range of 100 to 200 micro-inches in diameter to attain "excellent results". He also discloses that "if desired, a soft, porous, absorbent cover may be provided on the porous body member". FIG. 5 of Palnik appears to show such porous body member 36 with a porous absorbent cover 37. Again the plating material is gold and other precious metals which are plated upon continuous strips of copper-based alloy used for electrical contacts.
U.S. Pat. No. 4,750,981 issued Jun. 14, 1988 to H. W. Dalland et al. discloses an electroplating method which is not a brush or selective coating method, but is a method for coating discrete surfaces of physical bodies, particularly bodies having orifices in them wherein a portable chamber is provided for clamping onto the surface of the work piece at the point where the coating is to be provided, said chamber having within it an anode which is located closely adjacent to, but electrically isolated from the surface to be coated. It is said that the anode is typically a carbon anode or else an anode made of the metal which is to be coated upon another metal. A preferred embodiment of Dalland et al. shows a pair of containers each positioned over the end of an orifice, the interior surface of which is to be plated. Dalland states that his invention or apparatus is designed to be used where neither tank plating nor brush plating are practical. He further discloses that "brush coating is not suitable for applying certain desired chrome platings that, heretofore, have required dip tank-type solutions".
U.S. Pat. No. 4,853,099 issued Aug. 1, 1989 to G. W. Smith discloses an electroplating apparatus for rapidly electroplating a surface of a work piece by a so-called gap-type electroplating which is not brush or selective plating. Smith provides an anode in a shape and having a surface generally matching the shape and selective surface of the work piece being plated and provides a current flow between the anode and cathode established by the geometry of the anode surface as it relates to the work piece being plated. While gap plating can be accomplished in a tank and is often done in a plating tank, it is stated that it can also be accomplished by directing a plating solution into the gap between the anode and cathode as a current is applied between such two electrodes as long as a closed fluid flow can be maintained through the gap. The contribution of the Smith patent to the art of gap coating is to form a very narrow gap and then pump a large amount of plating solution to it so that the plating solution passes very quickly. It is stated that these ultra high flow rates allow high current densities which in turn cause rapid deposition of metal from the flowing plating solution. The preferred element for use in the gap coating process is, according to Smith, nickel. Smith discloses that chromium plating is not very effective in his apparatus because the increase of density of current does not increase the plating of the chrome. He therefore prefers to coat with nickel, which will deposit at a rate that increases substantially with increased current density. However, Smith does state in column 11, lines 24 through 26 that some of the features of his invention may also assist in providing some benefit for a chromium plating system. It appears from Smith's discussion that the reason for the effectiveness of his process may be because overheating of the electrolytic solution is prevented by passing the solution very quickly through the plating area at a high flow rate. Smith also discloses that this ultra high flow of the electrolytic solution assures the removal of gas bubbles, the maintenance of low temperature and the high solution pressure contact with the anode surface and the work piece surface which he believes increases the efficiency of his system. He discloses a work opening usually of between 0.05 inches and 1 inch, but apparently prefers the narrower gaps in order to provide a higher flow.
As will be evident from the discussion above, it has not been possible previously to provide successful hard chromium coatings by the brush plating method and as a result, the preciseness of coating, the convenience of coating at the work site as well as the portability of the necessary apparatus and the other advantages of brush or selective coating have not been available for the provision of hard chrome coatings, yet hard chrome coatings are one of the prime metallic coatings for the repair particularly of the bearings for shafts, shaft surfaces and the like. There has been a critical and long continuing need, therefore, to have a brush plating-type apparatus and procedure for plating with hard chromium.